1 In The Movement of Soil, Part 1, you learned about the processes both human and natural that cause soil erosion. There are other forces that cause the movement of soil and rock. Gravity is one of those major forces that pull rock fragments down slopes in events called mass movements. Mass movements vary in their speed, intensity, and fragment size. When mass movements occur rapidly, they tend to be the most destructive and dramatic. Rockfalls occur when rock fragments fall from steep cliffs. This is the fastest type of mass movement. The fragments may be as tiny as pebbles or as huge as giant boulders. Landslides occur when large amounts of loose rock combined with soil fall suddenly down a slope. The steepness of the slope may affect the amount and the size of the rock material in the landslide. Larger landslides tend to occur on steeper slopes and involve larger blocks of bedrock. If you live in California or in areas where highways run through mountains, then you may have witnessed small landslides. Earthquakes and volcanic eruptions may cause landslides. Less dramatic events like heavy rainfall and spring thaws may also produce conditions that cause landslides.2 Rocks and boulders are not the only material involved in mass movements. Mudflows, which occur in dry mountainous areas, contain large amounts of mud that move rapidly. Heavy rainfall that happens suddenly and volcanic eruptions can cause mudflows. The traveling mud churns and tumbles its way down slopes. It continues through valleys and eventually spreads out in a large fan-shape at the slope's base. Mudflows are often mistaken for landslides, especially in communities along hillsides. Heavy rainfall may also cause slumps. Overlying soil that has been over saturated from rain may lose friction with underlying rock. As a result, large blocks of soil and rock may break off and slide down the slope in one piece. The block follows along the curved slope of the surface, slipping over solid rock towards the bottom of the slope. Slumps usually occur on very steep slopes.3Solifluction and creep are the slowest types of mass movement. The majority of the rock material that moves on our planet's surface is moved by slow mass movements. Solifluction occurs when soil that has been saturated with water slips over hard frozen layers of subsoil. This type of movement tends to happen in areas where the subsoil is permanently frozen, like the Arctic or in mountainous areas. Warmer regions that have subsoils made of hard clay may experience solifluction. Creep is the slowest of all the mass movements and moves the most soil out of all the mass movements. The only way to detect soil creep is to observe the fences, buildings, and other surface objects that may be in that area. As the loosened soil moves slowly downslope, these objects will move along with the soil. Soil creep may be caused by water loosening rock particles; plants producing pressure as they grow that wedges in between rocks and soil; or burrowing animals that loosen rock material as they build their homes. Constant freezing and thawing may also cause creep. Once the particles are loosened, gravity takes hold, and the soil slowly moves downhill. Eventually, these particles may accumulate at the base of the slope forming piles of talus. Talus cones contain these particles and will continue to weather into smaller particles. These smaller particles are then carried by runoff into gullies and other waterways.